To the casual observer, Goldman Sachs's decision to let Nasdaq take over the day-to-day running of its US dark pool probably looks strange. It appears the US bank is handing over control of a critical function while Nasdaq is aiding a trading rival. But the deal is smart for both sides.

​Goldman is hardly handing over control. It will retain ownership and regulatory responsibility for the platform, paying the US exchange operator a fixed fee to provide the underlying technology, monitoring and surveillance of the dark pool, Sigma X, the 11th-largest platform of its kind in the US.